This invention relates to monopole/slot antennas, and more particularly such antennas where the height of the monopole is reduced by the addition of compensating features.
By way of background, monopole/slot antennas (also referred to herein as full height monopole/slot antennas) are well known in the art. Such antennas are characterized as including a radiating slot fed by a stripline and backed by a cavity. The cavity is formed by upper and lower ground planes with conductive fasteners, posts, or the like around its perimeter. the fasteners act as shorting pins, forming the cavity walls, and preventing the parallel wave guide mode of radiation from the edges of the ground planes. Thus, the ground planes and shorting pins define the cavity. The upper ground plane has a slot therein, and a stripline is located midway between the ground planes. A monopole extends from the stripline and through the center of the radiating slot, orthogonal to the upper ground plane. The monopole is excited by directly contacting the stripline feed. Typically, suitable terminals are provided with such antennas to excite one end of the stripline and load the other.
With monopole/slot antennas of this type, the resonant frequency is determined by the monopole and slot dimensions. Because it is desirable to eliminate cavity influence on the antenna, the cavity dimensions typically are chosen so its lowest resonant frequency is considerably above the antenna operating range. The characteristic impedance of the antenna is determined in part by the width of the stripline and the thickness of the cavity.
the design of such monopole/slot antennas is well known in the art. Thus, for a given desired frequency range, resonant frequency, and characteristic impedance, the dimensions of the cavity, stripline width, slot, and monopole may be easily determined from known handbooks or known calculations.
Monopole/slot antennas of this type have several desirable operating characteristics. Because the monopole and slot are representative of reciprocal structures (electric dipole versus magnetic monopole), complementary tuning effects result that broaden the overall impedance bandwidth. The monopole/slot antenna develops a highly directive cardioid radiation pattern in the antenna ground plane with a deep null maintained over a wide bandwidth and occuring in the direction of the loaded end of the stripline. This cardioid pattern results from the superposition of the monopole and slot patterns. The monopole radiates a uniform pattern with constant phase in the ground plane direction. The slot radiates a figure 8 pattern with a 180.degree. phase shift between lobes. The deepest null occurs when the monopole and slot are excited equally. Because the radiation patterns of the monopole and slot element do not change appreciably as they become "electrically short", the combined monopole/slot antenna maintains a good cardioid radiation pattern over a wide frequency range.
While the monopole/slot antenna has many advantages, it has a principle disadvantage. The monopole protrudes above the antenna body. Its height is nearly one quarter wave length at the antenna's center frequency, which is nearly 3 inches (7.62 cm) at one GHz. Moreover, the monopole is a wire or rod, and must be protected by a radome in most installations. However, in some applications, no protrusions from the mounting surface can be tolerated, and therefore in those applications the full height monopole/slot antenna cannnot be used.
A known modification of the full height monopole/slot antenna is known as the hybrid slot antenna. Such an antenna is described in a technical paper entitled "The Hybrid Slot, a Versatile Low-Profile Radiator With Small Reflection Coefficient" by Mayes and Cwik, Electrical Engineering Department, University of Illinois, the entirety of which is incorporated by reference. A discussion of monopole/slot antennas may be found in the technical paper entitled, "The Monopole-Slot: A Small Broad Band Unidirectional Antenna", by Mayes, Warren, and Wiesenmeyer, IEEE Trans., AP-20 No. 4, pages 489-493, July, 1972, the entirety of which is incorporated by reference. With the hybrid slot antenna, the height of the monopole is reduced by top loading. This top loading is accomplished by providing a disk or "top hat" at the top of the monopole such that the larger the disk, the greater the loading. While the advantage of top hat loading is to reduce the height of the monopole, thus overcoming the chief disadvantage of the full height monopole/slot antenna, such top loading produces certain undesirable characteristics by sacrificing electrical performance when the monopole height is reduced. Specifically, with the hybrid slot antenna, there is a sacrifice in impedance bandwidth and radiation pattern performance.
The present invention substantially overcomes the disadvantages of the full height monopole/slot antenna without significant sacrifice in impedance, bandwidth, and radiation patterns. Thus, with the present invention the height of the monopole is substantially reduced while maintaining the impedance and cardioid radiation characteristics over a wide bandwidth. This is accomplished by offsetting the stripline toward the slotted ground plane, and capacitively loading the slot. The reduced height monopole/slot antenna of the present invention is a wide bandwidth antenna that develops a highly directive cardioid radiation pattern. It has low volume, excellent form factor, and easy producibility. The wide bandwidth occurs as a low input VSWR over an extremely wide frequency range, and a cardioid pattern in the antenna ground plane with a deep null is maintained over a wide bandwidth. The monopole element of the present invention, which protrudes above the antenna in the full height monopole/slot antenna, is significantly reduced in height by electrical loading. Therefore, for many applications, the antenna of the present invention can be flush mounted with no protrusions above the mounting surface. Like the monopole/slot antenna, the reduced height antenna of the present invention eliminates the problem of changing element impedances as frequency changes. It has a very wide bandwidth of nearly constant input impedance.
Thus, with the antenna of the present invention, the height of the monopole is greatly reduced. Moreover, through the other modifications, it retains the excellent electrical characteristics of the full height monopole/slot antenna with the added advantages of a low profile. These modifications give the antenna performance that is considerably improved for many applications over those previously known.